Self-aligned multiple patterning (SAMP) techniques (such as self-aligned double patterning (SADP) or self-aligned quadruple patterning (SAQP)) are currently used in ultra-high density integrated circuits to provide an electrical interconnection system which includes multiple arrays of parallel metal lines disposed in several levels of dielectric layers within a semiconductor structure. The dielectric layers are typically interconnected through a system of metalized vias.
In such SAMP techniques, the metal lines are typically formed from a series of self-aligned mandrel spacers disposed on sidewalls of parallel mandrels within the semiconductor structure. During the process flow, the mandrels are pulled to form a pattern from the remaining self-aligned mandrel spacers. The pattern is then transferred down into the underlying layers of the semiconductor structure to ultimately form alternating parallel mandrel and non-mandrel metal lines in a dielectric layer.
In an SADP technique, this basic process is performed once during the formation of the metal lines, such that the pitch of the metal lines is half the pitch of the mandrels. In an SAQP technique, this basic process is performed twice during the formation of the metal lines, such that the pitch of the metal lines is a quarter the pitch of the mandrels.
However, formation of interconnect systems having large arrays of multiple parallel mandrel and non-mandrel metal lines in a Back-End-Of-Line (BEOL) process flow for a semiconductor fabrication often require the metal lines to have variable line widths. This kind of variability in line width is very difficult to achieve with a conventional SAMP process. This is particularly the case when the technology is in the 7 nanometer (nm) technology class and beyond or when the minimum pitch (i.e., the minimum distance between repetitive features in a semiconductor device structure) is less than or equal to 40 nm.
Moreover, the problem is exacerbated even further when the line widths and the variations in line widths are too small for conventional lithographic process to resolve. More specifically, a typical SAMP process can print small constant width metal lines, such as an array of 20 nm constant width metal lines. A typical lithographic process can print large variable width metal lines, such as an array of metal lines having widths of 80 nm and 100 nm. However, variable width metal lines having widths (e.g., below 40 nm width) that are too small for a lithographic process to resolve are difficult to achieve with either an SAMP or a lithographic process.
Accordingly, there is a need for a method of forming mandrel and non-mandrel metal lines that have variable line widths. Moreover, there is a need for forming such variable line widths when the widths of the metal lines are below the resolution of a conventional lithographic process.